Professionals usually want both, speed and capacity. Speed means more money, but you also need something to store your data in.
Here is a test with SSD as a Photoshop scratch disk, the gains are pretty substantial. However, if you have hundreds of GBs, or even several TBs of data, SSDs are way too expensive. Hence HDs are needed for storage.
I would say SSD+HD is the best possible setup. External HD is not as convenient, especially if it's USB 2.0. Thunderbolt devices are still very limited and expensive, so not necessarily an option for everyone. Sure, if you have TBs, then an external is needed but for more frequently used files, an internal HD along with an SSD is the sweet spot.